1. Field of the Invention
The invention relates to a method for computer aided design, the system therefore and a medium storing the method, more particularly relates to the method, the system, and the medium which provide calculating distances between elements mounted on each surface of a printed circuit board.
2. Description of the Related Art
A calculation of a distance between elements mounted on each surface of a printed circuit board (PCB), can make it possible to evaluate influences of noise caused by a possible creepage discharge. The distance is measured along the surfaces, the distance is referred to as the creepage distance hereinafter, and the distance should be evaluated whether to agree or not with requirements in specifications and/or standards.
The layout of elements formed or arranged on PCBs is usually designed with a computer aided design system, hereinafter referred to as CAD system. In case of use of the CAD system, the creepage distance can be calculated by a function of the CAD system, where the creepage distance is a distance measured along a surface of a PCB between an element mounted on a front surface of a PCB, the element is referred as to the front element hereinafter, and other element mounted on a rear surface of the PCB, the other element is referred as to the back element. And the resultant creepage distance is judged or evaluated whether it agrees or not with the requirements.
A conventional measurement is explained in detail with referring to FIGS. 19A and 19B, where FIG. 19A is perspective view and FIG. 19B is a side view. A distance LA between a front element 710 and a side 712 of a front surface 702 of a PCB 700, a thickness LC of the PCB 700, a distance LB between a back element 720 and the side 712 are measured respectively. And then a creepage distance between the two elements is gained by summing the distances LA and LB and the thickness LC. However, a following consideration on the path to measuring the distance is necessary in case of particular locations of the elements.
As shown in FIGS. 20A and 20B, a pair of elements 710 and 720, each of them being arranged on both surfaces of the PCB 700, normal lines NA and NB can be overlapped as shown in FIG. 20B. In this case of the locations of elements 710 and 720, the creepage distance can be gained as the method described above. However, in case of locations of elements 710 and 730, there is no common normal line being normal to the side 712. In this case, an operator must estimate a point α which gives the minimum sum of the distances DA and DB, where DA is a distance between the element 710 and the point α and DB is a distance between the element 730 and the point α. The selection of the point α is subjectively estimated and selected by the operator. The creepage distance, in this case, is given by the sum of DA, DB and the thickness LC of the PCB 700. Thus on the CAD system, a function to calculating a distance of two points selected by the operator is provided. The operator can gain the creepage distance by use of the function.
Other method of calculating of the creepage distance is disclosed in a Japanese publication of unexamined application 2005-10835. The method in the publication is disclosed as a function in an insulation-evaluating system. The system itself comprises a process transforming a three dimensional model to a correspondent approximate polyhedron model, a process of topologically recognizing the polyhedoron model, a process of recognizing a relation with adjacent parts, a process of recognizing and creating data of voltage system, a process of extracting a pair of voltage systems including a problem by rough check, a process of calculating “a creepage distance” between both the voltage systems, a process of checking the creepage distance by the information in a specification for insulation, and a process of outputting the information of the voltage systems including problems.
In the prior art shown in FIGS. 19A, 19B, 20A, and 20B, the creepage distance is obtained as the summation of the thickness of the PCB and the distances calculated by a computer between a turning point and points on the front and back elements, where these three points should be indicated by the operator. Thus till obtaining a result, the method in the prior art requires frequently operation by the operator, and imposes on the operator the serious burden and requires a long time. Further, the resultant creepage distance is not guaranteed as the path having the minimum distance between both elements, because the turning point and the points on the front and back elements are subjectively selected by the operator. Therefore, the method in the prior art involves a problem that the resultant distance as a creepage distance has serious error and a low reliability.
Furthermore, it is necessary to obtain all creepage distances for every possible combinations of a plurality of elements arranged on the both surfaces of PCB. Therefore, it will be vast time to calculate the every creepage distances even if the turning point and the points on the front and back element are set automatically instead of the operator, because the combination of the points for calculating the creepage distances are enormous. Subsequently to detecting front and back element not compatible with the requirement in the specification or a standard, the similar calculation must be repeated after the rearrangement of the front elements by the operator so that the elements become to be compatible with the requirement. Therefore, the prior art has problems such that the method imposes serious burden on the operator and requires a vast time for a whole design process.
In the method disclosed in the Japanese publication of unexamined application 2005-10835, the calculation of the creepage distance requires the process of transforming the three dimensional model to a correspondent approximate polyhedron model and so on. The calculation in the process of transforming and the like are too vast to obtain the creepage distances between the front and back element on the PCB. The disclosed method in the application is suitable for analyzing a object having a complicate structure. In case of an object having a simple structure such as a rectangular contour of PCBs, it is desired to obtain in a short time the distances between elements on the PCB by a simple calculation such as summation of a distance from an element to a side of the PCB, the thickness of the PCB, and the distance from the side to other element.